1. Field of the Invention
The present invention is about phased array antenna using gain switched multimode Fabry-Perot laser diode (FP-LD) and high-dispersion-fiber. Especially, the invention deals with the techniques that allow compact and low-cost system implementation for phased array antenna by adopting optical control and also allowing continuous time delay for each antenna in the array to induce phase difference.
2. Description of the Related Technology
Electrically controllable phased array antenna is attracting great attention in applications such as microwave communication and radar systems. However, practical implementations are very limited, because true time delay system to induce phase difference between antennas is too complicated.
On the other hand, since optical phased array antenna uses fiber based optical systems it has many advantages such as ability to induce time delay easily, immunity to electromagnetic interference (EMI), efficiency of bandwidth usage, and capability to produce light and compact systems.
FIG. 1 is a conventional phased array antenna structure diagram, which uses optical fiber grating as time delay line and compose of wavelength tunable laser (100), external modulator (110), 3 dB coupler (120a, 120b, 120c, 120d), optical fiber grating (130a, 130b, 130c, 130d), photodetector (140a, 140b, 140c, 140d), amplifier (150a, 150b, 150c, 150d), and antenna (160a, 160b, 160c, 160d).
In FIG. 1, optical power from wavelength tunable laser (100) is modulated by external modulator (110) which utilizes the electro-optics effect caused by RF (radio frequency) signals that are transferred to the antenna. The modulated power is then inputted to delay line of optical fiber grating (130a, 130b, 130c, 130d) through 3 dB coupler (120a, 120b, 120c, 120d).
Here, wavelength dependent time delay occurs due to the different reflection time for different laser wavelength. The light signal is then inputted to photodetector (140a, 140b, 140c, 140d) through 3 dB coupler (120a, 120b, 120c, 120d), where it is converted photo-electrically (optic-to-electric: O/E) into RF signal, and inputted into each elements of the antenna (160a, 160b, 160c, 160d).
However, the amount of time delay in the above configuration is dependent on the spacing of fiber grating. The advantage that this kind of methods for using optical fiber grating is it requires only a single light source and short length of optical fiber. However, it has the disadvantage that beam position of phased array antenna not being continuous.
FIG. 2 is a conventional phased array antenna, which uses high-dispersion-optical fiber and compose of wavelength tunable laser (200a, 200b, 200c, 200d), external modulator (210a, 210b, 210c, 210d), photodetector (220a, 220b, 220c, 220d), amplifier (230a, 230b, 230c, 230d), antenna (240a, 240b, 240c, 240d), laser control signal (250a, 250b, 250c, 250d), micro-signal source (260a, 260b, 260c, 260d), and high-dispersion fiber (270a, 270b, 270c, 270d).
In FIG. 2 system utilizes the phenomenal fact that optical fiber has wavelength dependent dispersion property. In this system, optical power of wavelength tunable laser (200a, 200b, 200c, 200d) is modulated by external modulator (210a, 210b, 210c, 210d) using RF signal, where it passes through high-dispersion fiber (270a, 270b, 270c, 270d), and then phase shifted RF signal is obtained through the photodetector (220a, 220b, 220c, 220d).
The time delay obtained in the above system is dependent on the amount of dispersion of the fiber, length of the fiber, and wavelength difference of the wavelength tunable laser. Therefore, in this case, since a multiplicity of wavelength tunable lasers and external modulators are required, it was difficult to implement systems at low cost.
FIG. 3 is a conventional dispersive and non-dispersive optical fiber based phased array antenna with a single light source and a single modulator. The system of this figure compose of wavelength tunable laser (300), external modulator (310), laser control signal (320), 1XN power splitter (330), dispersive fiber (340), non-dispersive fiber (350), photodetector (360), amplifier (370), and antenna (380).
In FIG. 3, instead of using a multiplicity of light sources and modulators as in FIG. 2, optical power is distributed by 1xc3x97N power splitter (330), and time delay is achieved by adjusting the lengths of dispersive fiber and non-dispersive fiber in the high-dispersion fiber portion. To make use of this method in implementation on practical systems, an additional temperature stabilizing system is required, because time delay difference arises due to different temperature property between dispersive fiber (340) and non-dispersive fiber (350).
FIG. 4 shows method for using conventional chirped fiber grating (CFG) which compose of pattern controller (400), wavelength tunable laser (410a, 410b, . . . , 410n), optical multiplexer (420), external modulator (430), circulator (440), CFG (450), wavelength demultiplexer (460), photodetector (470a, 470b, . . . , 470n), amplifier (480a, 480b, . . . , 480n), and antenna (490a, 490b, . . . , 490n).
This system uses the phenomenal fact that the reflection position in CFG (450) is dependent on the selected chirping rule. Here, RF signal modulates the output power from wavelength tunable laser (410a, 410b, . . . , 410n) at the external modulator (430), and the modulated signal is inputted to the circulator (440).
Output signal from the circulator (440) is reflected in the chirped fiber grating that is configured according to the wavelength, so that it has a time delay corresponding to the grating spacing. It again passes through the circulator (440) and then into photodetector (470a, 470b, . . . , 470n), and finally output as phase shifted RF signal. In time delay path using CFG (450), since the grating spacing varies linearly, change in time delay can also be adjusted continuously. However, this method requires wavelength stability and linearity of CFG (450) as well as a multiplicity of light sources.
Since the method from FIG. 4 requires a shorter length of fiber for time delay compare to that of FIG. 3, it does not need an additional temperature stabilizing system as in FIG. 3. However, because adequate CFG""s are not commercially available, there is a practical limitation in implementing this type of method.
As mentioned hitherto, phased array antenna system utilizing time delay by fiber grating, CFG, or dispersive fiber in the prior art requires essentially a multiplicity of wavelength tunable lasers and external modulators. In the case of FIG. 3, although it uses a single light source and a single external modulator, it requires a microwave source to modulate over the microwave band, over which the antenna operates. Hence, the overall system was difficult to build at a low cost.
Therefore, it is necessary to provide a simple and low-cost system for phased array antenna over the microwave band, applicable in the practical wave environment.
The main objective of the present invention is to resolve the aforementioned problems and, therefore, to provide an accurate low-cost phase array antenna system, which does not need costly external modulator and microwave signal source as in the prior art. Such system is available in the present invention by electrically controlling the phase of phased array antenna, while utilizing the features of optical system using the same method of optically controllable phased array antenna as in the prior art.
To achieve the aforementioned objective, the present invention is to provide a time delay characterized phased array antenna by first generating optical pulses by gain switching of multimode Fabry-Perot laser diode(FP-LD), and making them into optical pulse train with varied wavelengths using mode separation by high-dispersion fiber, then distributing the signal by power splitter, and passing it through each fiber of different lengths to cause time delay.
The above and other features and advantages of the present invention will be more clearly understood for those skilled in the art from the following detailed description taken in conjunction with the accompanying drawings, which form parts of this disclosure.